Titanated chromium-based catalysts to produce polyethylene

ABSTRACT

The present invention relates to a supported chromium-based catalyst titanated under specific conditions and used for the homopolymerisation or the copolymerisation of ethylene. The polyethylene obtained with this catalyst has high shear resirance and environmental stress crack resistance, and can be used for manufacturing films with improved tear properties.

This is a continuation of application Ser. No. 09/092,658, filed Jun. 5,1998, Now U.S. Pat. No. 6,200,980.

The present invention relates to a catalyst for producing a high densitypolyethylene having a broad molecular weight distribution, in order toobtain good processability and good physical and chemical properties. Inparticular, the good physical properties may be improved tear propertieswhen the polyethylene is made into films and/or improved environmentalstress crack resistance. The present invention further relates to aprocess for producing said catalyst and to the use of such a catalyst.

For polyethylene, and for high density polyethylene (HDPE) inparticular, the molecular weight distridution (MWD) is a fundamentalproperty which determines the properties of the polymer, and thus itsapplications. It is generally recognised in the art that the molecularweight distribution of a polyethylene resin can principally determinethe physical, and in particular the mechanical, properties of the resinand that the provision of different molecular weight polyethylenemolecules can significantly affect the Theological properties of thepolyethylene as a whole.

The molecular weight distribution can be completely defined by means ofa curve obtained by gel permeation chromatography. Generally, themolecular weight distribution (MWD) is more simply defined by aparameter, known as the dispersion index D, which is the ratio betweenthe average molecular weight by weight (Mw) and the average molecularweight by number (Mn). The dispersion index constitutes a measure of thewidth of the molecular weight distribution. For most applications, themolecular dispersion index varies between 10 and 30.

Since an increase in the molecular weight normally improves the physicalproperties of polyethylene resins, there is a strong demand forpolyethylene having high molecular weight. These high molecular weightmolecules, however render the polymer more difficult to process. On theother hand, a broadening in the molecular weight distribution tends toimprove the flow of the polymer when it is being processed at high shearrates. Accordingly, in applications requiring a rapid transformation ofthe material through a die, for example in blowing and extrusiontechniques, the broadening of the molecular weight distribution permitsan improvement in the processing of polyethylene at high molecularweight (high molecular weight polyethylenes have a low melt index, as isknown in the art). It is known that when the polyethylene has a highmolecular weight and also a broad molecular weight distribution, theprocessing of the polyethylene is made easier as a result of the lowmolecular weight portion while the high molecular weight portioncontributes to a good impact resistance for the polyethylene resin. Apolyethylene of this type may be processed using less energy with higherprocessing yields.

As a general rule, a polyethylene having a high density tends to have ahigh degree of stiffness. In general, however, the environment stresscrack resistance (ESCR) of polyethylene has an inverse relationship withstiffness. In other words, as the stiffness of polyethylene isincreased, the environment stress crack resistance decreases, and viceversa. This inverse relationship is known in the art as theESCR-rigidity balance. It is required, for certain applications, toachieve a compromise between the environmental stress crack resistanceand the rigidity of the polyethylene.

Polyethylene is well known in the art for use in making films.Typically, polyethylene films are blown or extruded through a die. Theblowing and extrusion of the thin film defines for the film a machinedirection in the direction of blowing or extrusion through the die andan orthogonal transverse direction. For many applications, thepolyethylene film is required to have a high tear strength, and inparticular a high isotropy in the tear strength between the machine andtransverse directions. As a result of the blowing or extrusiontechnique, the polyethylene polymer chains can become substantiallyaligned in the machine direction of the blowing or extrusion process.This can yield a significantly higher tearing strength in the transversedirection of the film as compared to the tearing strength in the machinedirection. There is generally a need for polyethylene films having goodtear properties for use in the manufacture of films, and in particular agood isotropy in the tear properties between the machine and transversedirections. A variety of catalyst systems are known for the manufactureof polyethylene. It is known in the art that the physical properties, inparticular the mechanical properties, of a polyethylene resin can varydepending on what catalyst system was employed to make the polyethylene.This is because different catalyst systems tend to yield differentmolecular weight distributions in the polyethylene produced. Thus forexample the properties of a polyethylene resin produced using achromium-based catalyst (i.e. a catalyst known in the art as a “Phillipscatalyst”) tend to be different from the properties of a productemployed using a Ziegler-Natta catalyst.

For the manufacture of polyethylene films, it is known that HDPE resinsmade using Ziegler-Natta catalysts have a good balance in their tearproperties between the machine and transverse directions. In particular,such resins made using Ziegler-Natta catalysts, and having what is knownin the art as a bimodal molecular weight distribution, have goodisotropic tear properties. Such a bimodal HDPE resin has a bimodaldistribution of the molecular weight of the high density polyethylenewhich is represented in a graph of the molecular weight distribution asdetermined for example by gel phase chromatography. The graph includesin the curve a “shoulder” on the high molecular weight side of the peakof the molecular weight distribution. Such a bimodal high densitypolyethylene consists of high and low molecular weight fractions inwhich the mixture of those fractions is adjusted as compared to amonomodal distribution so as to increase the proportion of highmolecular weight species in the polymer.

The production of high density polyethylene using just a chromium-basedcatalyst is thus desirable to enable the particular polyethylene productto be manufactured. The Encyclopedia of Polymer Science and Engineering,Volume 6, pages 431-432 and 466-470 (John Wiley & Sons, Inc., 1986, ISBN0-471-80050-3) and Ullman's Encyclopedia of Industrial Chemistry, FifthEdition, Volume A21, pages 501-502 (VCH Verlagsgesellschaft mbH, 1992,ISBN 3-527-20121-1) each discuss Phillips and Ziegler-Natta catalystsand the production of HDPE.

It is known in the art that in order to obtain the advantages of a broadmolecular weight distribution, it is necessary to polymerise an intimatemixture of polyethylene molecules prepared in a common manufacturingprocess, It is known in the art that it is not possible to prepare apolyethylene having a broad molecular weight distribution and therequired properties simply by blending polyethylenes having differentmolecular weights.

It has thus been proposed to carry out the polymerisation by a two stepprocess, using two reactors connected in series (GB-A-1233599;EP-A-057352; U.S. Pat. Nos. 4,414,369 and 4,338,424). In a first stepand in the first reactor a fraction of the high density polyethylene isproduced under specified conditions and in the following second step inthe second reactor a second fraction of the high density polyethylene isproduced using a different set of polymerisation conditions. In thetwo-step process, the process conditions and the catalyst can beoptimised in order to provide a high efficiency and yield for each stepin the overall process. The currently commercially employed two-stepprocesses suffer from the disadvantage that because two separate serialprocesses are employed, the overall process has a low throughput.

It has further been proposed to produce polyethylene with a broadmolecular weight distribution with a two-catalyst mixture of onesupported chromium catalyst and one Ziegler-Natta type catalyst(EP-A-0480376). This process suffers from the disadvantage that theZiegler-Natta catalyst requires a co-catalyst to give an activecatalytic system but the co-catalyst can influence the supportedchromium catalyst and in particular can detrimentally affect itsactivity.

It has also been proposed, for example in EP-A-661299, EP-A-647661 or WO95/33777 to use chromium-based catalysts for the production ofpolyolefins.

There is a need in the art for a process for producing polyethyleneresins suitable for blow molding having good environmental stress crackresistance (ESCR) and suitable for the manufacture of films having goodtear properties, which do not use a Ziegler-Natta catalyst, and inparticular which use a chromium-based catalyst.

It is known in the art to provide titanium in a chromium-based catalyst.Titanium can be incorporated either into the support for the chromiumcatalyst or into the catalytic composition deposited on the support.

Titanium can be incorporated into the support by coprecipitation orterprecipitation as is the case for cogel and tergel type catalystsdeveloped by Phillips Petroleum described for example in EP-A-352715.Cogel and tergel catalysts respectively have binary and ternarysupports.

Alternatively, titanium can be incorporated into the support byimpregnation of the support as described for example in U.S. Pat. No.4,402,864 and FR-A-2,134,743 or by chemisorption of a titanium compoundinto the support as described for example in U.S. Pat. No. 4,016,343.

Titanation of the catalytic composition has been disclosed in earlierpatent specifications.

U.S. Pat. No. 4,728,703 discloses that titanium can be incorporated intothe catalytic composition by adding to a composite liquid suspension, ofa carrier material (i.e. a support) and chromium trioxide, a titaniumcompound of the formula Ti(OR)₄.

U.S. Pat. No. 4,184,979 discloses that titanium can be incorporated intothe catalytic composition by adding at elevated temperature a titaniumcompound such as titanium tetraisopropoxide to a chromium-based catalystwhich has been heated in a dry inert gas. The titanated catalyst is thenactivated at elevated temperature.

The ethylene polymers obtained with all the above mentioned processes donot have satisfactory mechanical properties especially with regard tothe environmental stress crack resistance (ESCR).

Therefore there exists a need for a chromium-based catalyst capable ofproducing polyethylene resins for blow molding, having goodprocessability and good physical and chemical properties.

It is an aim of the present invention to provide a catalyst for thepolymerisation of ethylene to produce polyethylene having goodprocessability.

It is another aim of this invention to provide a catalyst for producingethylene with high environmental stress crack resistance.

It is yet another aim of the present invention to provide a process forproducing polyethylene using a chromium-based catalyst, with theresultant polyethylene resin being suitable for the manufacture ofpolyethylene films and in particular wherein the films have a goodbalance in their tear properties between the machine and transversedirections.

It is a further aim of the present invention to provide a catalyst forproducing polyethylene having the above described desired properties,said catalyst having a high activity.

These and other aims can be achieved with a supported titanatedchromium-based catalyst prepared under specific conditions, saidcatalyst being used for the production of high density polyethylene withimproved processability and physical and chemical properties.

The present invention provides a process for preparing a titanatedchromium-based catalyst for the production of high density polyethylene,by polymerising ethylene, or copolymerising ethylene and analpha-olefinic comonomer comprising 3 to 10 carbon atoms, whichcomprises the steps of;

a) providing a silica-containing support having a specific surface areaof at least 400 m²/g;

b) depositing a chromium compound on the support to form achromium-based catalyst;

c) dehydrating the chromium-based catalyst to remove physically adsorbedwater by heating the catalyst at a temperature of at least 300° C. in anatmosphere of dry, inert gas;

d) titanating the chromium-based catalyst at a temperature of at least300° C. in an atmosphere of dry, inert gas containing a titaniumcompound of the general formula selected from R_(n)Ti(OR′)_(m) and(RO)_(n) Ti(OR′)_(m) wherein R and R′ are the same or different and area hydrocarbyl group containing from 1 to 12 carbon atoms, n is 0 to 3, mis 1 to 4 and m+n equals 4, to form a titanated chromium-based catalysthaving a titanium content of from 1 to 5% by weight, based on the weightof the titanated catalyst and

e) activating the titanated catalyst at a temperature of from 500 to900° C.

The present invention further provides a catalyst for the production ofhigh density polyethylene, by polymerising ethylene, or copolymerisingethylene and an alpha-olefinic comonomer comprising 3 to 10 carbonatoms, the catalyst comprising a silica-containing support having aspecific surface area of at least 400 m²/g, a chromium compounddeposited on the support, and a titanium compound deposited on thesupport and comprising from 1 to 5% by weight Ti, based on the weight ofthe titanated catalyst.

The present invention also provides a process for producingpolyethylene, in the presence of a chromium-based catalyst for theproduction of high density polyethylene, by polymerising ethylene, orcopolymerising ethylene and an alpha-olefinic comonomer comprising 3 to10 carbon atoms, the catalyst comprising a silica-containing supporthaving a specific surface area of at least 400 m²/g, a chromium compounddeposited on the support, and a titanium compound deposited on thesupport and comprising from 1 to 5% by weight Ti, based on the weight ofthe titanated catalyst.

The present invention further provides the use of the catalyst of theinvention in the production of high density polyethylene for providing ahigh environmental stress crack resistance and a low incidence of meltfracture when melted and subjected to rotational shear at varyingspeeds.

The present invention further provides the use, for increasing theisotropy of the tear properties of films made from polyethylene resins,of the catalyst system of the invention.

The present invention is predicated on the surprising discovery of thepresent inventor that, in the production of polyethylene resins, aparticular chromium-based catalyst having a minimum specific surfacearea of a silica-containing support and which has been de-hydrated andthe surface titanated prior to or during the process of the activationof the catalyst at elevated temperature,can unexpectedly yield highdensity polyethylene having a very high environmental stress crackresistance (ESCR) and a low melt fracture index and is able to improvethe tear balance of a polyethylene film made from the polyethyleneresin.

The silica-containing support material used in the catalyst of thisinvention can be any catalytic support known in the art. The support isan inorganic, solid, particulate porous material inert to the othercomponents of the catalyst composition and to: any other activecomponents of the reaction system. Thus, suitable supports are inorganicmaterials, such as silica either alone or in combination with othermetallic oxides, e.g., silica-alumina or silica-titania.

The support used in this invention has a large surface area of at least400 m²/g, preferably from 450 to 600 m²/g and more preferably from 475to 550 m²/g. The support preferably has a pore volume greater than 1cm³/g, more preferably from 1 to 3 cm³/g, yet more preferably from 1.3to 2.5 cm³/g. It is preferred that the support be dried prior to anychromium species being deposited onto it.

Known chromium-containing compounds capable of reacting with the surfacehydroxyl groups of the silica-containing supports can be utilised todeposit the chromium thereon. Examples of such compounds includechromium nitrate, chromium trioxide, chromate esters such as chromiumacetate, chromium acetylacetonate and t-butyl chromate, silyl chromateesters and phosphorous-containing esters. Preferably, chromium trioxideis used.

A preferred chromium-based catalyst may comprise from 0.5 to 3% byweight of chromium, preferably about 1% by weight of chromium, on acatalyst support, such as a composite silica and titania support.

A particularly preferred chromium-based catalyst for use in the presentinvention comprises the catalyst “catalyst 1”, having a surface area of450 m²/g, a pore volume of around 1.5 cc/g and a chromium content ofaround 1 weight % based on the weight of the chromium-containingcatalyst. The support comprises a silica support. Other preferredsilica-supported catalysts have a specific surface area of 500 m²/g andrespective pore volumes of 2 and 3 cc/g. Another preferred catalyst witha silica support has a specific surface area of 450 m²/g and a porevolume of 1.5 cc/g.

The support is dried by heating or pre-drying of the support with aninert gas prior to use thereof in the catalyst synthesis, in the mannerknown to those skilled in the art, e.g. at about 200° C. for from 8 to16 hours.

The chromium-based catalyst can be prepared by dry mixing or non-aqueousimpregnation but is preferably prepared by impregnation of silica withan aqueous solution of a soluble chromium compound such as CrO₃.

The supported chromium-based catalyst is subjected to a pretreatment inorder to dehydrate it by driving off physically adsorbed water from thesilica or silica-containing support i.e. chemically adsorbed water inthe form of hydroxide (—OH) groups bonded to the —Si—O— framework of thesupport need not be removed. The removal of physically adsorbed wateravoids the formation of TiO₂ as a product from the reaction of waterwith the titanium compound subsequently introduced during the titanationprocedure, as described below. The dehydration step is preferablycarried out by heating the catalyst to a temperature of at least 300° C.in a fluidised bed and in a dry inert atmosphere of, for example,nitrogen. The dehydration step is preferably carried out for 0.5 to 2hours.

In a next step, the supported chromium-based catalyst is loaded with atitanium compound. The titanium compound may be of the formulaR_(n)Ti(OR′)_(m) or (RO)_(n) Ti(OR′)_(m) where R and R′ are the same ordifferent and can be any hydrocarbyl group containing 1 to 12 carbonatoms, n is 0 to 3, m is 1 to 4 and m+n equals 4. Preferably, thetitanium compound is a titanium tetraalkoxide Ti(OR′)₄ where R′ can bean alkyl or a cycloalkyl group each having from 3 to 5 carbon atoms. Thetitanation is performed by progressively introducing the titaniumcompound into the stream of dry, inert non-oxidising gas describedhereabove in the dehydration step. In the titanation step, thetemperature is, as for the dehydration step, maintained at at least 300°C. Preferably, the titanium compound is pumped as a liquid into thereaction zone where it vaporises. This titanation step is controlled sothat the titanium content of the resultant catalyst is from 1 to 5% byweight, and preferably from 2 to 4% by weight, based on the weight ofthe titanated chromium-based catalyst. The total amount of titaniumcompound introduced into the gas stream is calculated in order to obtainthe required titanium content in the resultant catalyst and theprogressive flow rate of the titanium is adjusted in order to provide atitanation reaction period of 0.5 to 1 hour.

After the introduction of the titanium compound has been terminated atthe end of the reaction period, the catalyst is flushed under the gasstream for a period of typically 0.75 hours.

The dehydration and titanation steps are performed in the vapour phasein a fluidised bed.

The titanated catalyst is then subjected to an activation step in dryair at an elevated activation temperature for at least 6 hours. Theactivation temperature preferably ranges from 500 to 900° C., and ismost particularly around 650° C. Improved ESCR is obtained when theactivation temperature is around 650° C. The atmosphere is progressivelychanged from nitrogen to air, and the temperature is progressivelyincreased, from the titanation step to the activation step.

The resultant titanated chromium-based catalyst has a very highactivity.

In the preferred polymerisation process of the present invention, thepolymerisation or copolymerisation process is carried out in the liquidphase, the liquid comprising ethylene, and where required analpha-olefinic comonomer comprising from 3 to 10 carbon atoms, in aninert diluent. The comonomer may be selected from 1-butene, 1-hexene,4-methyl 1-pentene, 1-heptene and 1-octene. The inert diluent ispreferably isobutane. The polymerisation process is typically carriedout at a polymerisation temperature of from 85 to 110° C. and at apressure of from 20 to 45 bars. Preferably, the temperature ranges from95 to 105° C. and the pressure from 40 to 42 bars to produce polymerresins of high ESCR. Preferably, the temperature ranges from 90 to 94°C. and the pressure is at a minimum of about 24 bars to produce filmswith improved tear properties.

Typically, in the polymerisation process the ethylene monomer comprisesfrom 0.5 to 8% by weight, typically around 6% by weight, of the totalweight of the liquid phase. Typically, in the copolymerisation processthe ethylene monomer comprises from 0.5 to 8% by weight and thecomonomer comprises from 0 to 4% by weight, each based on the totalweight of the liquid phase.

A chemical reducing agent, such as a metal alkyl, may be introduced intothe polymerisation reaction. The metal alkyl may comprise triethylaluminium (TEAI) in an amount of around 0.5 ppm by weight based on theweight of the inert diluent. This can be used when lower activationtemperatures for the catalyst have been employed.

Whilst the operating conditions, such as the temperature and pressure ofpolymerization in the reactor, and the catalyst's preparationconditions, such as the surface area of the support, obviously have aninfluence on the properties of the polymer, titanation of the catalystunder the specific conditions described above improves the ESCR, allother factors being substantially equal.

The titanated chromium-based catalyst is introduced into thepolymerisation reactor. The alkylene monomer, and comonomer if present,are fed into the polymerisation reactor. In the preferred process of thepresent invention, the polymerisation or copolymerisation process iscarried out in a liquid-full loop reactor; after a residence time in thereactor of 0.5 to 2 hours, and preferably of about one hour, thepolyethylene is recovered and transferred to one or more settling legswhere the concentration in solids is increased by gravity. The solidcontent in a loop reactor is typically 30 to 40% by weight; theconcentration in a settling leg can be up to 60% by weight. Thepolymerisation product of high density polyethylene is discharged fromthe settling legs and separated from the diluent which can then berecycled.

The polyethylene obtained with the catalyst of this invention has abroad molecular weight distribution (MWD) which is represented by thedispersion index D of typically from 12 to 23 and a high densitytypically from 0.948 to 0.960 g/cm³.

It is surprisingly observed that the polyethylene obtained with thecatalyst of this invention has much higher environmental stress crackresistance (ESCR) and a much lower melt index than those obtained usingthe processes and catalysts of the prior art as summarised above, whilekeeping similar melt indices and densities. The polyethylene obtained inaccordance with the invention also has a very high shear resistance (SR)defined as HLMI/MI2 where HLMI is the high load melt index measured at190° C. and under a load of 21.6 kg and MI2 is the melt index measuredat 190° C. under a load of 2.16 kg, both with the ASTM D-1238 standardmethod. The high shear resistance can result in suppression of the meltfracture phenomenon.

The following Examples are given to illustrate the invention withoutlimiting its scope.

EXAMPLES 1 TO 7

These Examples illustrate polyethylene with better ESCR.

A silica support was impregnated with 1 wt % chromium by the followingsteps. 60 g of silica were dried by heating for one hour at 130° C., andthen for 3 hours at 500° C. The silica was allowed to cool for 45minutes in a desiccator. 50 g of the dried silica were placed undervacuum for 30 minutes. A solution was prepared by dissolving 3.498 g ofchromium acetylacetonate in 250 ml of acetone. The solution was added tothe dried silica drop by drop until the silica was completely saturated.Then the rest of the solution was added in a slow continuous stream. Thesaturated silica was agitated for 2 hours, then left overnight. Theacetone was evaporated off at a temperature of 70° C. and under apressure decreasing from 400 to 200 mbars until a fine, dry powder wasobtained. The catalyst was then completely dried in an oven at 110° C.overnight.

This chromium-treated support was then introduced into an activatorvessel incorporating a fluidised bed, flushed under nitrogen and thetemperature was raised from room temperature to 300° C. The dehydrationstep was then carried out at this elevated temperature for 2 hours.After the dehydration step, titanium tetraisopropoxide, stored underanhydrous nitrogen, was progressively injected in the bottom of theactivator vessel incorporating the fluidised bed. The amount of titaniumtetraisopropoxide injected was calculated in order to give the requiredtitanium content in the resultant catalyst and the flow thereof wasadjusted in order to continue the injection to complete the desiredlevel of titanation in around 30 minutes. After the injection wascompleted, the catalyst was flushed under nitrogen for around 45minutes. Nitrogen was then progressively switched to air and thetemperature was raised to the activation temperature of around 650° C.for the susequent activation step. In the activation step, the titanatedchromium-based catalyst was maintained at the activation temperature for6 hours. At the end of the activation step, the temperature wasprogressively decreased to 350° C. At continued cooling from 350° C. toroom temperature, the catalyst was flushed under nitrogen.

EXAMPLE 8

The polymerisation of ethylene was carried out using the same procedureas in Examples 1 to 7 but the dehydration and titanation temperatureswere both 400° C.

Table I specifies for each of Examples 1 to 8, the nature and thespecific surface area of the support, the titanium weight percent, andthe dehydration, titanation and activation temperatures.

The polymerization of ethylene was carried out in one liquid-full loopreactor in accordance with the polymerization process described aboveand in the presence of the catalyst of each Example prepared under theconditions specified above.

The polymerisation conditions for each of Examples 1 to 8 are alsospecified in Table I. In Example 7, a cocatalyst TEAL (triethyaluminium)was employed in the polymerisation process in the amount specified inTable I.

For each of Examples 1 to 8 the pressure was 42 bars; the diluent wasisobutane and the comonomer was 1-hexene.

The weight percent of ethylene and 1-hexene are given in Table I.

The resulting melt index MI2, shear resistance SR, density, molecularweight distribution as represented by the dispersion index D, ESCR andmelt fracture are also given in Table I. The melt indices were measuredwith the ASTM D-1238 method at a temperature of 190° C. and under a loadof 2.16 kg for MI2 and 21.6 kg for HLMI. The ESCR values referred toherein are the Bell ESCR F50 values as determined in accordance withASTM D-1693-70, Procedure B.

All the Examples shown in Table I have a surface area of the supportlarger than 450 m²/g, are dehydrated and susequently titanated at anelevated temperature and are activated at a temperature of about 650° C.All the resultant polyethylenes exhibit simultaneously the desiredproperties of high stress crack resistance ESCR and low incidence ofmelt fracture when melted and subjected to rotational shear at varyingspeeds. Examples 3 to 7 for which results are available at rotationalspeeds of 20, 40 and 60 rotations per minute showed no fracture.

Comparative Examples 1 and 2

The polymerization of ethylene was carried out using the same procedureas in Examples 1 to 7 but the catalyst was not titanated.

Comparative Examples 3 to 5

The polymerization of ethylene was carried out using the same procedureas in Examples 1 to 7 but the catalyst's titanation was obtained byimpregnation as described in U.S. Pat. No. 4,402,864 andtriethylaluminum was used as a cocatalyst.

Comparative Examples 6 and 7

The polymerization of ethylene was carried out using the same procedureas in Examples 1 to 7 but the chromium catalyst was not titanated andtriethylaluminum was used as a cocatalyst. The surface area of thecatalyst was only 316 m²/g for each of these Comparative Examples.

Comparative Example 8

The polymerization of ethylene was carried out using the same procedureas in Examples 1 to 7 but the catalyst's support had a small surfacearea (308 m²/g) and contained titania. The catalyst was not titanated.Triethylaluminum was used as a cocatalyst.

Comparative Example 9

The polymerization of ethylene was carried out using the same procedureas in Examples 1 to 7 but the catalyst's support had a small surfacearea (280 m²/g), the chromium-based catalyst was not titanated, andtriethylaluminum was used as a cocatalyst.

Tables II specifies for each of Comparative Examples 1 to 9, the sameinformation as for Examples 1 to 8.

In Comparative Examples 1 and 2, the surface area is large (492 m²/g)but there is no titanation; the ESCR is high but at a rotational speedof 60 rotations per minute, melt fracure occured in four tests inComparative Example 1 and in three tests in Comparative Example 2. Nomelt fracture occured at rotational speeds of 20 and 40 rotations perminute.

In Comparative Examples 3 to 5 and 8 where the titanium is in thesupport, the ESCR is very low.

In Comparative Examples 6, 7 and 9 where there is no titanium, the ESCRis very low.

TABLE I Example 1 Example 2 Example 3 Example 4 Example 5 Example 6Example 7 Example 8 Support Silica Silica Silica Silica Silica SilicaSilica Silica Surface area m²/g 458 458 484 484 484 484 484 467 %Titanium by weight 2.3 2.3 4 4 4 4 4 4 Dehydration temperature (° C.)300 300 300 300 300 300 300 400 Titanation temperature (° C.) 300 300300 300 300 300 300 400 Activation temperature (° C.) 650 650 650 650650 650 650 650 Polymerisation conditions Temperature (° C.) 106 105 104102 102 102 102 102 TEAL (ppm) 0 0 0 0 0 0 1 0 Ethylene (wt %) 5 4.9 5.35.2 6 5.1 5.1 5.9 1-hexene (wt %) 0.08 0.17 0.24 0.15 0.17 0.29 0.250.11 Hydrogen (mole %) 0.17 0.17 0.09 0.09 0.09 0.09 0.04 0.11Properties of polyethylene MI2 (g/10′) 0.28 0.17 0.31 0.285 0.27 0.310.27 0.23 Shear response SR 84 96 86 101 96 85 97 106 Density (g/cm³)0.9581 0.9548 0.9544 0.9582 0.9581 0.952 0.952 0.958 Dispersion index D13.6 14.9 13.2 13.6 13.8 15.2 15.7 14 Bell ESCR F50 (hrs) 63 179 175 7676 169 153 61 100% Antarox Melt fracture 20 rpm n.a. n.a. 0 0 0 0 0 n.a.40 rpm n.a. n.a. 0 0 0 0 0 n.a. 60 rpm n.a. n.a. 0 0 0 0 0 n.a. n.a.:not available

TABLE II Comparative examples Example 1 Example 2 Example 3 Example 4Example 5 Example 6 Example 7 Example 8 Example 9 Support Silica SilicaSilica Silica Silica titan. by Silica Silica Silica Titania Silica-alum.titan. by titan. by impregn. phosphate impregn. impregn. Surface aream²/g 492 492 490 490 490 316 316 308 280 % Titanium by weight 0 0 4 4 40 0 2 0 Dehydration temperature (° C.) — — — — — — — — — Titanationtemperature (° C.) — — — — — — — — — Activation temperature (° C.) 650650 650 650 650 600 700 650 650 Polymerisation conditions Temperature (°C.) 106 106 99 99 99 108 102 106 106 TEAL (ppm) 0 0 0.8 0.8 0.8 0.8 0.80.3 0.3 Ethylene (wt %) 4.5 3.9 4.1 3.7 4.4 4.9 5.2 4.9 4.7 1-hexene (wt%) 0.06 0.1 0.11 0.16 0.22 0.03 0.07 0.09 0.1 Hydrogen (mole %) 0.470.77 0.09 0.09 0.09 0 0 0 0.9 Properties of polyethylene MI2 (g/10′)0.15 0.23 0.19 0.25 0.245 0.22 0.2 0.32 0.15 Shear response SR 90 82 123114 116 82 109 78 126 Density (g/cm³) 0.9566 0.9552 0.9558 0.9542 0.9511 0.9580 0.9587 0.9567 0.9573 Dispersion index D 11 9.8 13.8 13 13.2 119.2 10.2 11.7 Bell ESCR F50 (hrs) 84 145 31 45 58 40 <18 n.a. n.a. 100%Antarox Melt fracture 20 rpm 0 0 n.a. n.a. n.a. n.a. n.a. n.a. n.a. 40rpm 0 0 n.a. n.a. n.a. n.a. n.a. n.a. n.a. 60 rpm 4 3 n.a. n.a. n.a.n.a. n.a. n.a. n.a. n.a.: not available

EXAMPLES 9 TO 12

These Examples Illustrate the Better Tear Properties of Films Made withthe Polyethylene of the Invention.

In order to demonstrate the process of the present invention in which adehydrated titanated and activated chromium-based catalyst was employed,a number of runs to copolymerise ethylene to form high densitypolyethylene were performed. In each of the Examples, a liquidcomprising ethylene, 1-hexene and the balance isobutane as an inertdiluent was fed into a polymerisation reaction zone. Hydrogen was alsofed into the reaction zone. The temperature in the reaction zone rangedfrom 90 to 94° C. and the pressure ranged from 38 to 42 bars. In Example12, a metal alkyl, in the form of triethyl aluminium (TEAI), was alsointroduced into the reaction zone as a reducing agent. Table IIIsummarizes the processing conditions of Examples 9 to 12.

The catalyst system was also fed into the polymerisation reaction zone.In Examples 9 to 12, the chromium-based catalyst system comprised thecatalyst identified as “catalyst 1” which had been activated at anactivation temperature of around 650° C. The catalyst had beendehydrated and then titanated prior to activation by mixing the catalystwhich a titanium-containing compound comprising titaniumtetraisopropoxide. In Examples 9 to 12, the percentage of titaniumpresent in the catalyst comprised from 2 to 4 weight % based on theweight of the catalyst system.

The dehydration, titanation and activation steps were carried out asfollows. The chromium-based catalyst was then introduced into anactivator vessel incorporated a fluidised bed, flushed under nitrogenand the temperature was raised from room temperature to 300° C. Thedehydration step was then carried out at this elevated temperature for 2hours. After the dehydration step, titanium tetraisopropoxide, storedunder anhydrous nitrogen, was progressively injected into the bottom ofthe activator vessel incorporating the fluidised bed. The amount oftitanium tetraisopropoxide injected was calculated in order to give therequired titanium content in the resultant catalyst and the flow thereofwas adjusted in order to continue the injection to complete the desiredlevel of titanation in around 30 minutes. After the injection wascompleted, the catalyst was flushed under nitrogen for around 45minutes. Nitrogen was then progressively switched to air and thetemperature was raised to the activation temperature of around 650° C.for the subsequent activation step. In Example 12 a lower activationtemperature of around 550° C. was employed and the reactivity of thepolymerisation process was enhanced by the addition of TEAL to thereactants as specified in Table III. In the activation step, thetitanated chromium-based catalyst was maintained at the activationtemperature for 6 hours. At the end of the activation step, thetemperature was progressively decreased to 350° C. At continued coolingfrom 350° C. to room temperature, the catalyst was flushed undernitrogen.

Table III also summarises the properties of pellets of the polyethyleneresins produced in Examples 9 to 12 and also the properties of films ofvarying thickness produced from those polyethylene resins. For Examples9 to 12, the high load melt index (HLMI) ranged from 11.8 to 15.1 g/10min and the melt index MI₂ ranged from 0.07 to 0.086 g/10 min. The meltindex MI₂ and the high load melt index HLMI were determined using theprocedures of ASTM D 1238 using respective loads of 2.16 kg and 21.6 kgat a temperature of 190° C. The MI₂ and the HLMI are broadly inverselyindicative of the molecular weight distribution of the polymer. Theshear response for the polyethylene resins of Examples 9 to 12, being aratio of the HLMI and the MI₂ values, ranged from 156 to 189. Thedensity values for the polyethylene resins produced in accordance withExamples 9 to 12 are specified in Table III and range from 0.958 to0.951 g/cc.

The polyethylene resins of Examples 9 to 12 were processed to producefilms having a thickness of 10 microns, 20 microns and 40 microns. TableIII shows the tear properties of those films. The tear strengths areexpressed in N/mm. The tear strength in the machine direction (MD) andthe tear strength in the transverse direction (TD) were measured usingthe procedures of ASTM D1922. For each of the films, the ratio betweenthe TD and MD values was calculated and these are specified in TableIII.

It will be seen from Table III that for each of the three filmthicknesses employed in Examples 9 to 12 of 10, 20 and 40 microns, theTD/MD ratio is relatively low, being a maximum value of 5.7 for the 10micron film of Example 9. This indicates a relatively good isotropy inthe tear strength of the films. For example, it will be seen for 40micron films of Examples 9 to 12 that the TD/MD ratio only ranges from aminimum value of 0.8 to a maximum value of 1.5, indicating a relativelyhigh degree of isotropy in the tear strength in the machine andtransverse directions.

Comparative Example 10

In Comparative Example 10, the process of Examples 9 to 12 was repeatedusing the same chromium-based catalyst “catalyst 1” and using the sameactivation temperature of 650° C. and using the same processingconditions in the polymerisation process. However, in ComparativeExample 10 the chromium-based catalyst was not subject to titanationprior to the activation step. The equivalent properties of thepolyethylene resins produced in Comparative Example 10, and theproperties of the films produced from those resins, are shown in TableIII. It may be seen that the polyethylene resin produced in ComparativeExample 10 has a significantly lower shear response of 96 as compared tothe shear responses of the resins of Examples 9 to 12. This lower shearresponse value indicates reduced processability of the polyethyleneresins. In addition, it may be seen that for each of the three filmthicknesses of 10, 20 and 40 microns of Comparative Example 10, theTD/MD ratio is significantly higher for any given film thickness thanfor each of Examples 9 to 12. This demonstrates that the use of atitanated catalyst during activation significantly and unexpectedlyimproves the isotropy in the tear properties of a polyethylene film.

Comparative Examples 11 and 12

Comparative Examples 11 and 12 demonstrate in Table III the typicalproperties of a polyethylene resin, and films having thickness of 20 and40 microns produced therefrom, produced by a copolymerisation processusing a Ziegler-Natta (Z-N) catalyst. It will be seen that theproperties of the resins of Examples 9 to 12 are substantially similarto those of the resins produced using Ziegler-Natta catalysts. Moreover,it may be seen that the tear properties of the films, in particular theisotropy of the tear strength of the films in the machine and transversedirections, are substantially similar for the films of Examples 9 to 12and as compared to those produced from the resins of ComparativeExamples 11 and 12.

Comparative Example 13

Comparative Example 13 demonstrates in Table III the typical propertiesof a polyethylene resin, and films having a thickness of 10,20 and 40microns produced therefrom, produced by a copolymerisation process usinga chromium-containing catalyst having a composite silica and titaniasupport. In Comparative Example 13, the catalyst employed, “catalyst 2”,had an average pore radius of 190A, a pore volume of around 2.1 cc/g anda chromium content of around 1 wt % based on the weight of thechromium-containing catalyst. This catalyst contained 3 wt % titanium(in the titania in the support) based on the total weight of thecatalyst. The catalyst was not subjected to a prior dehydration andtitanation process before activation at around 550° C. It will be seenfrom Table III that for each of the film thicknesses employed the ratioof the tear strength in the transverse direction and the machinedirection exhibited greater anisotropy than for the Examples inaccordance with the invention.

Thus the present invention provides a process for making polyethyleneresins which can be employed to produce films having good tearproperties but with the process employing a chromium-based, as opposedto a Ziegler-Natta, catalyst.

TABLE III EXAMPLES COMPARATIVE EXAMPLES 9 10 11 12 10 11 12 13 CATALYST1 1 1 1 1 Z-N Z-N 2 % Ti 2 4 4 4 0 — — 3 ACTIVATION 650 650 650 550 650— — 550 Temp (° C.) PROCESS CONDITIONS Temp (° C.) 90 93 90 94 103 — —89 Alkyl (TEA1) (ppm) 0 0 0 0.5 0 — — 0 Ethylene (kg/h) 9.4 9 9 10 9 — —9 Hexene (cc/h) 748 495 705 945 549 — — 612 Hydrogen (N1/h) 0 0 0 0 80 —— 0 Isobutane (kg/h) 26 26 26 26 26 — — 26 PELLET PROPERTIES HLMI(g/10′) 15.1 13.4 11.8 13.5 12.5 6.3 9.5 14.7 MI2 (g/10′) 0.08 0.0860.07 0.082 0.13 0.037 0.05 — SHEAR RESPONSE 189 156 169 165 96 170 190107 DENSITY (g/cc) .949 .951 .949 .948 .950 .948 .956 .950 FILMPROPERTIES 10 μm MD 9 15 20 10 6.7 — — 8 TD 51 25 26 41 116 — — 136TD/MD ratio 5.7 1.7 1.3 4.1 17.3 — — 17 20 μm MD 16 20 30 19 9.8 13 23 7TD 50 30 36.2 50 90 52 43 136 TD/MD ratio 3.1 1.5 1.2 2.6 9.2 4 2 19.440 μm MD 23 30 38.9 29 7.6 22 19 16 TD 35 23.5 30.1 36 33 51 25 76 TD/MDratio 1.5 0.8 0.8 1.2 4.3 2.5 1.2 4.8 *Z-N = Ziegler-Natta

What is claimed is:
 1. A titanated chromium-based catalyst for theproduction of high density polyethylene, by polymerizing ethylene, orcopolymerizing ethylene and an alpha-olefinic comonomer comprising 3 to10 carbon atoms, the catalyst comprising a silica-containing supporthaving a surface area of at least 400 m²/g, a chromium compounddeposited on the support, and a titanium compound deposited on thesupport, the catalyst comprising from 1 to 5% by weight Ti.
 2. Thecatalyst according to claim 1 wherein the surface area is from 450 to600 m²/g.
 3. The catalyst according to claim 1 wherein the surface areais from 475 to 550 m²/g.
 4. The catalyst according to claim 1 whereinthe catalyst comprises from 2 to 4% by weight Ti.
 5. A process forpreparing polyethylene in the presence of a titanated chromium-basedcatalyst for the production of high density polyethylene, orcopolymerizing ethylene and an alpha-olefinic comonomer comprising 3 to10 carbon atoms, the catalyst comprising a silica-containing supporthaving a specific area of at least 400 m²/g, a chromium compounddeposited on the support, and a titanium compound deposited on thesupport, the catalyst comprising from 1 to 5% by weight Ti.